1. Field of the Invention
This invention relates to a phosphor and a radiation image storage panel employing the same, and more particularly, to a divalent europium activated barium fluorohalide phosphor and a radiation image storage panel employing the same.
2. Description of the Prior Art
Recently, it has been found that a divalent europium activated barium fluorohalide phosphor (BaFX:Eu.sup.2+, in which X is at least one halogen selected from the group consisting of Cl, Br and I) absorbs and stores a portion of radiation energy when exposed to a radiation such as X-rays, and emits light in the near ultraviolet to blue region when excited with an electromagnetic wave within a wavelength region of 450-900 nm after exposure to the radiation, that is, the phosphor shows stimulated emission (the peak wavelength of the stimulated emission is within the region of approx. 385-405 nm, depending upon the kind of halogen which is a component of the phosphor). In particular, the divalent europium activated barium fluorohalide phosphor has been paid much attention and investigated as a stimulable phosphor employable for a radiation image storage panel (i.e., stimulable phosphor sheet) which is used in a radiation image recording and reproducing method utilizing the stimulability thereof.
A radiation image storage panel has a basic structure comprising a support and at least one phosphor layer provided on one surface of the support, which comprises a binder and a stimulable phosphor dispersed therein. Further, a transparent protective film is generally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock.
The radiation image recording and reproducing method using the above-described radiation image storage panel comprising the stimulable phosphor is a very advantageous method replacing the conventional radiography. As described, for example, in U.S. Pat. No. 4,239,968, the method involves steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; exciting the stimulable phosphor with an electromagnetic wave such as visible light and infrared rays (hereinafter referred to as "stimulating rays") to sequentially release the radiation energy stored in the stimulable phosphor as light emission (stimulated emission); photoelectrically detecting (reading out) the emitted light to obtain electric signals; and reproducing a visible image from the electric signals on a recording material such as a photosensitive film or on a display device such as CRT.
In the radiation image recording and reproducing method, a radiation image can be obtained with a sufficient amount of information by applying a radiation to the object at considerably smaller dose, as compared with the case of using the conventional radiography. Accordingly, this method is of great value especially when the method is used for medical diagnosis.
The operation of reading out the radiation energy stored in the radiation image storage panel in the abovedescribed method is generally carried out by the steps of scanning the panel with a laser beam (stimulating rays) to sequentially excite the stimulable phosphor so as to release the radiation energy stored therein as light emission and detecting the emitted light by a photosensor such as a photomultiplier.
In the last step of the read-out operation, the light which is continuously emitted by the stimulable phosphor of the radiation image storage panel after terminating the excitation with stimulating rays (namely, afterglow of stimulated emission) causes the decrease of S/N ratio of the resulting image. More in detail, the afterglow given by the phosphor particles other than the phosphor particles aimed to excite is detected as the light emitted by the aimed ones in the case that the phosphor gives afterglow in a relatively high ratio to the amount of the stimulated emission. As a result, the image provided by the panel comprising such a stimulable phosphor tends to be deteriorated on the image quality (sharpness, density resolution, etc.).
The afterglow characteristics of the panel varies depending not only on the employed stimulable phosphor but also on the intensity of stimulating rays, the scanning speed, etc., in the case of using a laser beam as the stimulating rays. In practical use, the influence of afterglow on the image quality further varies depending upon the detecting procedure of stimulated emission. However, it is of great value to improve the afterglow characteristics of the panel which give an adverse effect to the image quality, even if the improvement is not so high.
The radiation image recording and reproducing method using the radiation image storage panel comprising the stimulable phosphor is very advantageous as described above, and the method is desired to show the sensitivity thereof as high as possible. The sensitivity of the radiation image storage panel to a radiation generally increases as the luminance of stimulated emission of the phosphor employed in the panel increases. Accordingly, it is desired that the luminance of stimulated emission of the phosphor employed in the panel is as high as possible.
The above-mentioned divalent europium activated barium fluorohalide phosphor is improved in the luminance of stimulated emission by incorporating a specific amount of sodium halide thereinto as described in Japanese Patent Provisional Publication No. 59(1984)-56479. The phosphor containing sodium halide gives stimulated emission of high luminance, but tends to show the increased afterglow of stimulated emission.
Further incorporation of a specific amount of calcium and/or strontium into the phosphor brings about the improvement of afterglow characteristics as described in Japanese Patent Applications No. 58(1983)-24731, No. 59(1984)-11762 and No. 59(1984)-11763 (whole content of which is described in U.S. Patent Application Ser. No. 687,574 or European patent application No. 84116448.6). That is, a divalent europium activated barium fluorohalide phosphor containing calcium and strontium in addition to sodium halide, which has the following formula: EQU (Ba.sub.1-a-b,Ca.sub.a,Sr.sub.b)FX.multidot.cNaX':xEu.sup.2+
in which each of X and X' is at least one halogen selected from the group consisting of Cl, Br and I; a and b are numbers satisfying the conditions of 0.ltoreq.a.ltoreq.10.sup.-1 and 0.ltoreq.b.ltoreq.10.sup.-1 and a+b.noteq.0, provided that a+b is not higher than 5.times.10.sup.-2 in the case of ab.noteq.0; and c and x are numbers satisfying the conditions of 0&lt;c.ltoreq.2.0 and 0&lt;x.ltoreq.0.2, respectively, gives stimulated emission of high luminance and is improved in the afterglow characteristics.